1. Field of the Invention
This invention relates to a process for cracking heavy liquid hydrocarbons such as, for example, crude oil, heavy oil, bottom residue of vacuum distillation and the like which contain varying amounts of carbon residues.
2. Description of the Prior Art
Various processes are known for cracking to as high as asphalt to produce hydrogen, ethylene, propylene, butadiene, benzene, toluene, xylene and the like at high temperatures.
In commercial processes for producing petrochemical raw materials, there are often used light hydrocarbons such as natural gas, refinery off gas, naphtha, and light oil produced by atmospheric pressure distillation, but high-boiling oils such as crude oil, light oil from reduced pressure distillation and the like are used only to a limited extent.
The main reason is that the heavier the raw material, the easier carbon and tar-like materials are formed as by-products upon cracking. The resulting by-product attach to and deposit in the cracking apparatus and quenching devices for the cracked gas and therefore extended running periods such as 1000 hours or more become impossible.
A number of processes have been proposed for suppressing the formation of carbon and tar-like materials, and for preventing them from attaching to and depositing in the apparatus, but these conventional processes have various drawbacks such as high investment requirements, increase in necessary heat energy requirements which may be provided such as steam and the like, and decrease in the energy recovered from the process. No method of eliminating these drawbacks and providing for continuous operation utilizing high boiling oils has yet been discovered.
Processes for cracking hydrocarbons in the presence of molten salts or molten metals are known. In these prior art processes, the molten salt is usually used as a heat transfer medium. Hydrocarbons such as high-boiling oils are blown into a molten salt to receive the quantity of heat necessary for cracking at an elevated temperature. According to this type of cracking process, the sum of the sensible heat required for heating crude oil or a combination of crude oil and a diluting agent such steam up to a cracking temperature, heat of vaporization of crude oil and decomposition reaction heat of crude oil amounts to about 1000 Kcal. per Kg. of crude oil.
The quantity of molten salt circulating as a heat transfer medium amounts to many times the weight of crude oil. Therefore, the quantity of the molten salt in the apparatus is very large. From the view point of safety, it is not desirable to keep such a large quantity of a molten salt at high temperature in an oil cracking plant where large quantities of combustible and inflammable materials are always present.
In addition, circulating such a large quantity of a molten salt renders design of apparatus and its operation very difficult. Additionally, large amounts of energy are needed to circulate large amounts of molten salt.
No completely satisfactory method for avoiding these disadvantages has yet been discovered, although several have been proposed.
Among such attempted processes, those disclosed in U.S. Pat. Nos. 3,192,018 and 3,210,268 are relatively similar to the present invention. However, these processes still suffer from the above-mentioned disadvantages from a commercial point of view. For example, U.S. Pat. No. 3,192,018 discloses alkali metal chlorides as the molten salt. The salt passes together with crude oil through a reactor at a high velocity. In this process a large quantity of a molten salt must be used as a heat transfer medium to impart the required quantity of heat. One result is that none of the heat transfer medium can used in a mist form. Instead liquid drops of a fairly large drop size such as about 1000 microns in size are employed.
The processes mentioned above can prevent carbon from depositing on the reaction vessel wall, but the resulting carbon particles are of such a large size that they can not be removed by the water gas reaction under the cracking conditions. Therefore, the molten salt can not be reused unless the carbon is removed by combustion of carbon or by other treatment.
Japanese Patent Publication Nos. 19244/1972 and 8791/1975 disclose processes for preventing carbon from depositing in a quenching device by employing molten metals or molten heavy metal salts. In particular, the former procedure is quite similar to the present invention, but utilizes a molten metal as a flowing entrainment medium. The particle size of the molten metal is large. Therefore, it has drawbacks similar to those of U.S. Pat. No. 3,192,018. Further the metals and their compounds can not be subjected to the water gas reaction so that other treatments are necessary to remove carbon.